The present invention relates to a process for the production of Ziegler-Natta catalysts, and in particular to the production of such a catalyst comprising titanated magnesium chloride.
U.S. Pat. No. 4,636,486 discloses a chemical preparation process for a magnesium halide supported Ziegler-Natta catalyst. The most active anhydrous magnesium chloride is obtained when its x-ray diffraction spectrum is characterised by a broadened halo appearing at a lattice distance (d) within the range of from 2.56 to 2.95 Angstroms. This may be compared to a very crystalline magnesium chloride which exhibits an intense diffraction line at a lattice distance (d) of 2.56 Angstroms. Thus this prior patent document discloses that a polymerisation catalyst based on anhydrous magnesium dichloride has improved activity.
The chemical process disclosed in this document is a multi-step procedure and the activity of the catalyst is not particularly high.
U.S. Pat. No. 4,650,778 discloses the production of metal halide particles useful as olefin polymerisation catalyst precursors by vaporising the metal halide and then condensing it in the presence of a diluent.
It is also known for example from EP-A-0654444 and EP-A-0654485 to produce powder catalysts based on magnesium chloride, titanium chloride and at least an electron donor by using a plasma torch which vaporises the compounds introduced into the plasma torch and the vapour is condensed to form a fine magnesium chloride powder covered with titanium chloride. The powder is employed as a catalyst in the polymerisation of alpha-olefins.
While those latter two prior specifications, both of which are in the name of the present applicant, disclose the production of catalysts having good activity in the polymerisation of alpha-olefins, nevertheless there is the demand for such catalysts produced by a chemical process yet with high activity.
EP-A-0015099 discloses a variety of different methods for producing a Ziegler-Natta catalyst comprising titanated magnesium chloride. The process includes pulverising with a phosphorous compound a magnesium halide until a halo appears in the x-ray diffraction spectrum, and thereafter treating the pulverised product with liquid titanium halide to fix a titanium compound thereon.
BE-A-744221 discloses the production of Ziegler-Natta catalysts in which anhydrous magnesium chloride and titanium oxychloride are reacted together to form a product in which the x-ray diffraction spectrum has an intense line at a lattice distance (d) of 2.56 Angstroms
EP-A-0344755 discloses the production of Ziegler-Natta catalysts in which a magnesium compound is transformed into a magnesium dihalide by using an halogenating agent and thereafter the magnesium dihalide is reacted with a titanium or vanadium compound. It is disclosed that the magnesium dihalide has a halo in the x-ray diffraction spectrum.
For each of EP-A-0015099 and EP-A-0344755, the magnesium dihalide which is reacted with the titanium compound has a halo in the x-ray diffraction spectrum, indicating that the magnesium dihalide is not crystalline but rather is present as a disordered phase.
The present invention provides a process for producing a Ziegler-Natta catalyst, the process comprising reacting an organo magnesium compound with a halogenated compound to produce crystalline magnesium halide having an intense diffraction line in an x-ray spectrum thereof at a lattice distance within the range of from 2.56 to 3.20 Angstroms and titanating the magnesium halide by mixing the magnesium halide with a titanium compound whereby the titanated magnesium halide has, in an x-ray spectrum thereof, a halo appearing at a lattice distance within the range from 2.56 to 3.30 Angstroms.
The crystalline magnesium halide which is titanated in accordance with the process of the invention has a high degree of crystallinity as represented by the crystalline magnesium halide having an intense diffraction line at a lattice distance of from 2.56 to 3.20 Angstroms in the x-ray powder spectrum thereof. For example, when the halide is chloride, the intense diffraction line is at a lattice distance of 2.56 Angstroms. When the halide is bromide or iodide, the intense diffraction line is respectively at 2.93 or 3.19 Angstroms. Thus in accordance with the present invention a crystalline magnesium halide is titanated and in the titanation step the highly crystalline nature of the magnesium halide is destroyed to produce a titanated compound having a high degree of disorder, as reflected by the appearance of a halo in the x-ray diffraction spectrum instead of an intense diffraction line. In contrast, some of the known methods described hereinabove titanate not a crystalline magnesium halide, but a magnesium halide which already has a high level of disorder and thus exhibits already a halo in the x-ray spectrum. The present inventors have found that by titanating a crystalline magnesium halide to produce a titanated magnesium halide having disorder as represented by the halo in the x-ray spectrum, improved catalyst activity is achieved.
The magnesium halide preferably comprises magnesium chloride. It may alternatively comprise magnesium bromide or magnesium iodide.
The organo magnesium compound may comprise a magnesium organo-halide such as phenyl magnesium chloride or a magnesium alkoxide such as magnesium di(methyl 2 pentyloxide) or magnesium diethoxide.
The halogenated compound may comprise a substituted or unsubstituted silicon chloride, such as silicon dichloride diethoxide ((C2H5O)2SiCl2); diphenyl silicon dichloride ((C6H5)2SiCl2); phenyl silicon trichloride (C6H5SiCl3); triphenyl silicon chloride (C6H5)3SiCl) dimethyl silicon dichloride ((CH3)2SiCl2) or silicon tetrachloride. Typical chemical reactions for synthesising the magnesium chloride product are the reaction of phenyl magnesium chloride with diethoxy silicon dichloride; the reaction of magnesium dimethyl 2 pentyloxide) with diphenyl silicon dichloride ; or the reaction of magnesium diethoxide with dimethyl silicon dichloride, diphenyl silicon dichloride or silicon tetrachloride. For each reaction, the synthesis forms, in addition to the magnesium chloride, an electron donor. Thus the magnesium dichloride synthesis yields in-situ generation of an electron donor e.g. diphenyl diethoxy silane, diphenyl di(methyl 2 pentoxy) silane, dimethyl diethoxysilane or diethoxy dichlorosilane. This can assist in the formation of a highly crystalline magnesium chloride having a very intense diffraction line at a lattice distance of 2.56 Angstroms in the x-ray powder spectrum thereof. The electron donor is retained in the magnesium chloride support for the Ziegler-Natta catalyst. This in turn yields a silicon content for the support when the halogenated compound includes silicon.
Typically, the magnesium chloride synthesis is carried out by reacting the organomagnesium compound with the halogenated compound in an organic solvent at a temperature of from 20 to 60xc2x0 C. for a period of from 3 to 72 hours. Typically, the molar ratio of magnesium in the organomagnesium compound to chlorine in the chlorine compound is from 0.5 to 2. The organic solvent may comprise tetrahydrofuran (THF), heptane, xylene, dichloroethane or other suitable organic solvents.
The magnesium dichloride preferably is separated from the organic solvent, for example by a centrifuge, and is then washed with an organic solvent.
The magnesium chloride product as synthesised is then titanated, preferably by pouring the magnesium chloride into liquid titanium chloride. The resultant mixture is preferably held at a temperature of from 20 to 135xc2x0 C. for a period of from 0.5 to 2 hours. Thereafter, the titanated magnesium chloride may be separated from the titanium tetrachloride e.g. by filtration or by a centrifuge and washed with an organic solvent.
The present invention also provides a process for polymerising an olefin in the presence of the catalyst produced in accordance with the invention.
The present invention will now be described with reference to the following non-limiting Examples.